Paypal and multiple commerce sites

Hi, please can someone clarify for me.

I am in the middle of installing several separate multisites with Marketpress installed along with Pro sites. Each on independent domains.

Is my understanding correct in that I will have trouble with Paypal express payments completing correctly if I am using the same email (paypal account) on each of the different domains?

Or

Does Pro sites or Marketpress notify Paypal of the return url so that transactions complete in full and correctly? If so, do I need to create a return page or does Marketpress or Pro site handle this?

NB: Apologies, I would test this for an answer but I am not far enough with the install to checkout completely yet.